This invention relates to a process of using radiant energy combined with a radiation curable adhesive to bond components to glass where direct irradiation proves impossible. More particularly, the process utilizes the light scattering properties of glass to convey light energy along an axis generally perpendicular to the incident light, thus permitting cure of adhesive in shadow areas. Accordingly, the process can be used to bond a variety of components to glass utilizing radiant curing systems in areas not directly accessible to the applied light. The process derives particular utility in the bonding of light opaque end caps to fluorescent tubes.
Basically, there are four main sources of radiant energy: gamma rays, high energy electrons, neutrons, and ultraviolet. Each of these sources has its respective advantages and disadvantages. The use of radiant energy to cure polymeric coatings or adhesives is of fairly recent origin, and it is only in the last several years that the two most commercially attractive sources, ultraviolet and electron beam, have been developed for use in industry.
Typically, radiation curable compositions comprise various reactive components which cure by polymerization through free radical or ionic mechanisms. Each component is intended to perform a specific function in both the uncured composition and the cured product. The components include,
(1) a reactive low-to-medium weight polymer, generally referred to as an oligomer, which imparts primary performance characteristics to the cured product;
(2) monofunctional and polyfunctional reactive monomers which can contribute to the degree of crosslinking required in the cured product and otherwise function as a reactive diluent to adjust the viscosity of the formulation to a level suitable for application; and
(3) various non-reactive, specialty compounds such as fillers, colorants, slip agents and release agents, which are added for various end-use properties.
A characteristic feature of such 100 percent reactive compositions is the substantial absence of conventional inert volatile solvents. Even though inert, such solvents often have disadvantages of flammability, toxic hazards to workers, pollution of atmosphere and high cost. Such solvent-free systems provide high-performance formulations which can be cured at high line speeds by exposure to actinic radiation or ionizing radiation in either inert or oxygen-containing atmospheres.
Since ultraviolet light is one of the most widely used types of radiation because of its relatively low cost, ease of maintenance and low potential hazard to industrial users, rapid photo-induced polymerizations utilizing UV light rather than thermal energy for the curing of adhesives offer several significant advantages. First, faster curing materials off substantial economic benefits. Furthermore, heat sensitive material can be safely coated or bonded and cured with UV light without the use of thermal energy which could damage the substrate. Additionally, the essentially solvent free media reduces the necessity for expensive and time consuming pollution abatement procedures. Accordingly, UV curing systems, largely due to the above-identified advantages, have been met with increased usage in industrial settings.
While industrial use of UV curing systems has increased over the years, their use has been limited to cases where direct application of UV light to a substrate was necessary to effectuate cure. Accordingly, it is an object of the present invention to provide a new and improved use of conventional curing systems which utilizes the light transmitting properties of glass to convey UV light energy along an axis perpendicular to the incident UV light to cure an adhesive in an area not directly accessible to the applied UV light. It is a further object of the present invention to disclose a process using light and light activated adhesive compositions to secure light opaque end caps to fluorescent tubes.